Significance This article reports the discovery of a heretofore unknown icosahedral quasicrystal created by the detonation of the first nuclear device at Alamogordo, NM, on 16 July 1945 (the Trinity test). Like all quasicrystals, the new example violates crystallographic symmetry rules that apply to ordinary (periodic) crystals. It was found in a sample of red trinitite, a combination of glass fused from natural sand and anthropogenic copper from transmission lines used during the test. The new quasicrystal is the oldest extant anthropogenic quasicrystal known, whose place and moment of origin are known from the historic records of the Trinity test. The thermodynamic/shock conditions that formed it are roughly comparable to those that formed natural quasicrystals recently found in meteorites.